Cutout of plane with rounded corners
If i understood the question you need to curve the edges

Select the side verts and Press Ctrl+Shift+B to bevel

Select the edge and subdivide both edges

Select the face and press Shift+S and select cursor to selected

Select the single vert and press Shift+S selection to cursor

Repeat to other side also.

Select both the verts and Press Ctrl+Shift+B to bevel and with the scroll wheel increase the verts

Select the face and delete or extrude it.